1. Field of the Invention
The present invention relates to detecting small holes in packages. In particular, the present invention relates to use of tracer gases to detect leaks in packages, especially packages containing consumable products.
2. Discussion of Background
A vast array of products are marketed in closed packages. These products include consumable products, such as processed and unprocessed foods, herbs, spices, condiments, food supplements, medicines and beverages, and other consumer products such as batteries, film, and toiletries. In addition to their primary functions of containing and protecting the products from impact, packages provide barriers to bacteria, moisture and oxygen that might degrade or corrode the product. They carry advertising and instructions for use; they facilitate handling and stacking.
It is especially important to ensure the safety of food products. Many packaged non-food items also deteriorate as a result of leakage or contact with air. Assuring food product safety is very costly. Costs are incurred in processing the food prior to packaging, in adding preservatives and in expiration dating and in recovering and disposing of spoiled foods.
A complicating factor in assuring product safety is the sheer number of packages involved. Consumables are usually sold in relatively small quantities, except for commercial use. Another complicating factor is the number of different types, sizes, and styles of containers. Consumables can be packaged in aluminum or galvanized steel, glass bottles and plastic bottles with metal or plastic lids, rigid plastics or flexible plastics, metallicized plastics, and pasteboard.
Because of the scope of the problem of package inspection, typically only a limited number of packages are inspected in order to draw a statistical conclusion about the integrity of the package.
Some methods are available for testing packages. Air leak testing involves pressurizing the package by injecting air and measuring the decay in pressure caused by leaks. In compression testing, a filled, sealed package is placed on a flat surface and pressure is applied while an observer looks for leaks. Compression testing causes weak packages to fail but does not damage normal packages. Distribution testing simulates the events likely to occur during the package distribution process, including vibration, drop testing, and compression. Other methods include incubating at increased temperatures to stimulate microbial growth, viewing under polarized light to inspect plastic seals for defects or entrapped food, backlighting of empty packages to reveal holes, using X-rays to locate glass shards within glass jars, and measuring eddy currents to sense the position of metal components of packages. Most testing is destructive, must be done by skilled personnel and requires removing sample packages from the production line. These methods are inefficient, expensive and wasteful.
Holes down to approximately 100 .mu.m in size can be seen by the naked eye. Therefore, visual inspection is the most commonly-used testing method. The presence of visible holes, cuts, tears, dents, etc. indicates that the integrity of the package has been compromised. However, visual inspection is prohibitively expensive when 100% testing is required. Furthermore, small holes or crevices known as microleaks--holes less than approximately 100 .mu.m in size--cannot be found by visual inspection. Even such small holes provide pathways for bacterial contamination, leakage of air into the package, and, for some packaged items, slow leakage of the contents of the package. At present, microleaks can be identified only by destructive techniques. Nondestructive testing would offer the ability to evaluate every package produced rather than just a small sampling.
Helium and other gases, including sulfur hexafluoride (SF.sub.6), are used in industry as tracer gases to detect leaks in pressure vessels (Asai, et al., U.S. Pat. No. 5,163,315; Comell, et al., U.S. Pat. No. 3,585,845), fluid processing systems (Fries, U.S. Pat. No. 4,328,700) and steam condensers (Pelletier, et al., U.S. Pat. No. 4,226,113). The gas will pass through cracks or holes in pipe walls and is then subject to detection on the opposing side of the wall. Typically, in these industrial applications, the device to be tested is taken off-line, its orifices sealed and a small quantity of the tracer gas injected. If no trace of the gas appears on the outside of the wall, the device is presumed to be without holes or cracks. Then the device is placed back on line. Leak testing is generally done periodically, every year for example.
Helium is preferred in industry because its atoms are very small and low in mass, it has a high thermal velocity, and therefore can penetrate and move quickly through tiny openings or cracks in thick metal piping. Helium leak testing has also been used with food packages on a limited basis.
The inert gases, including helium, are not suitable for use in routine package testing. For example, because helium is chemically inert, it is difficult to detect, especially at low concentrations in air. Helium can be detected using a mass spectrometer, but mass spectrometers are relatively complex, delicate, and expensive instruments, limited in sensitivity to about 1 part per million (ppm). Routine package testing using helium as a tracer would therefore require large amounts of helium inside each package. Because of its low solubility in water and most other solvents, including edible oils, helium for leak-testing would necessarily be provided by replacing the air inside the package with helium. Helium has a low molecular weight and a low boiling point, thus, there is no known practical way of recapturing such packaged helium for re-use. Finally, because of its low molecular weight, helium readily diffuses through many intact solid materials, including common organic materials such as rubber, paper, plastic film, and so forth, some of which are favored for packaging consumables.
Them remains a need for a method for detecting small holes in packages that does not have the drawbacks of injecting helium into the container and yet provides increased assurance of the integrity of the package and is suitable for use in 100% product testing.